Pattern etching is a staple of semiconductor manufacturing. A substrate is commonly exposed to a plasma of reactive ions and neutrals to etch a pattern into a surface of the substrate. Such processes are typically used to etch a pattern into a substrate that is subsequently used in photolithographic patterning of semiconductor substrates. The substrate is usually glass or quartz, with a layer of chromium and/or molybdenum-doped silicon nitride on one side. The layer is covered with an anti-reflective coating and a photosensitive resist, and patterned by exposure to patterned UV light. Exposed portions of the resist are dissolved, and the underlying chromium layer is patterned by plasma etching.
During plasma etching, a plasma is generally formed adjacent the substrate. Reactive ions and radicals from the plasma react with the substrate surface, removing material from the surface. The rate of material removal, or etching, at a location on the substrate surface is proportional to the density of reactive species adjacent to that location. Due to microloading, variation in aspect ratio, plasma effects, and chamber effects, uniformity of the density of reactive species across the surface of a substrate often varies, resulting in variation of etch rate across the substrate. In many cases, etch rate is observed to be higher near the center of a substrate and lower near the periphery.
Prior methods of addressing etch rate uniformity include chemical methods of etch rate control, thermal methods of controlling precursor temperature and thermal profile of the plasma, and electromagnetic methods featuring electrodes placed at different locations within the chamber. There remains, however, a need for methods and apparatus that influence the density profile of a plasma in a dynamic, adjustable way.